Where's it leaking ?
Around the threads, or inner ball seal ?
My method.
1) Clean, clean, clean
2) Read instructions.
3) Tighten to proper torque.
You're probably going to say you did all those things.
I have one on my car and highly recommend them.
Easiest fix would be to tighten slightly more.
Worst case, drain the oil and follow steps 1-3